1. Field of the Invention
This invention relates to polyurethane elastomers and to their preparation and is more particularly concerned with polyurethane elastomers derived from modified 4,4'-methylenebis(phenyl isocyanate) and with processes for their preparation.
2. Description of the Prior Art
A wide range of thermoplastic non-cellular polyurethanes is currently available for use in the preparation of articles by injection molding, extrusion and like techniques. These elastomers are generally prepared by reaction of a diisocyanate, a polymeric diol and a difunctional extender. These materials require care both in the processes used for their manufacture and in the molding processes to which they are subjected in producing fabricated articles.
Thus the materials in question are subject to partial degradation upon exposure to high temperatures for excessive periods during manufacture or processing. In addition they are extremely sensitive, at the elevated temperatures employed in manufacture and or processing, to reaction with atmospheric moisture with subsequent undesirable effects on physical properties. The sensitivity to moisture is increased in direct proportion to the temperature to which the polymer is exposed during manufacture and processing. It is accordingly desirable to develop methods for reducing or minimizing these problems.
Further, it is extremely important that the properties of any particular polyurethane elastomer be maintained within clearly specified boundaries in order to ensure uniformity from batch-to-batch in processing behaviour and characteristics as well as in physical properties of the fabricated articles. One of the factors which can materially effect the reproducibility of properties of these elastomers is the variation from batch-to-batch in reactivity of the diisocyanate employed in their preparation. It is accordingly also desirable to devise methods for eliminating or markedly reducing the variation in reactivity of said diisocyanate.
It is an object of this invention to provide polyurethane elastomers which can be processed by molding, extrusion and the like at temperatures lower than those hitherto employed thereby reducing sensitivity to degradation by heat and atmospheric moisture. It is a further object to reduce significantly the variation in reactivity of diisocyanates thereby facilitating reproducibility of properties of polyurethane elastomers derived therefrom.
These objectives are achieved by employing as the diisocyanate a 4,4'-methylenebis(phenyl isocyanate) which has been pre-reacted with a trace of a monohydric alcohol.
The use of such monohydric alcohols, commonly referred to in the art as chain terminators, for other purposes is well known; see, for example, U.S. Pat. Nos. 3,350,361; 3,384,623; 3,425,973; 3,483,167; 3,761,439; and 3,823,111. These references show carrying out various polyurethane forming reactions either in the presence of the monohydric alcohol or with addition of the monohydric alcohol at the end of the reaction to terminate the latter. Monohydric alcohols have also been employed widely as capping or masking agents, for example, to cap the free isocyanate groups in an isocyanate-terminated prepolymer which is to be stored in combination with a chain extender to provide a composition which, upon heating at any desired time, will regenerate the free isocyanate groups and lead to formation of a polyurethane.
U.S. Pat. No. 3,294,713 describes the reaction of a mixture of polymethylene polyphenyl polyisocyanates with a monohydric alcohol in sufficient quantity for the latter to react with from 2 to 50 percent of the isocyanate groups present in the polyisocyanate. The polyisocyanate so treated is said to have improved compatability with the other components employed in the formation of polyurethanes and to give rise to more flexible cellular products.
To the best of our knowledge it has not previously been suggested that 4,4'-methylenebis(phenyl isocyanate) be modified in the manner described herein nor that such a modified isocyanate be employed in the preparation of polyurethane elastomers. It has also not been recognized that the resulting elastomers would have the highly desirable properties set forth below.